Both forgot and forgotten are used as the past participle of forget: Many have already forgot (or forgotten) the hard times of the Depression. Only forgotten is used attributively: half-forgotten memories.

forget

v.

Old English forgietan, from for-, used here with negative force, "away, amiss, opposite" + gietan "to grasp" (see get). To "un-get," hence "to lose" from the mind. A common Germanic construction (cf. Old Saxon fargetan, Old Frisian forjeta, Dutch vergeten, Old High German firgezzan, German vergessen "to forget"). The literal sense would be "to lose (one's) grip on," but that is not recorded in any Germanic language. Related: Forgetting; forgot; forgotten.